In German Published Patent Application No. 43 13 331, the camshaft position is detected here using a sensor wheel, which is permanently attached to the camshaft, and a camshaft sensor. The sensor wheel is provided with a reference mark which rotates with the sensor wheel and thus with the camshaft. With each revolution of the reference mark, the camshaft sensor emits a signal corresponding to a specific angular position of the camshaft. Accordingly, the camshaft position is only precisely determinable at the points in time of signal emission, i.e., only when the reference mark passes by the camshaft sensor. Since the rotational movements of crankshaft and camshaft are coupled, in the event of failure of the crankshaft sensor the crankshaft position is determined or estimated on the basis of the angular positions detected by the camshaft sensor. Only if the camshaft position can be precisely determined can the crankshaft position also be precisely determined. Between the individual angular positions detected by the camshaft sensor, the crankshaft position is estimated via extrapolation from the previously detected angular positions of the camshaft. Therefore, taking into account the gear transmission ratio between crankshaft and camshaft, an extrapolation over 180° of crankshaft angle takes place here.
For lack of space, the sensor wheel attached to the camshaft is relatively small so that the number of reference marks situated on the sensor wheel may not be significantly increased. Moreover, the maximum number of the reference marks is limited by the resolution capability of the camshaft sensor. Overall, as a rule, the resolution of the camshaft sensor signal is much lower than the resolution of the crankshaft sensor signal. Due to this fact, the method described in German Published Patent Application No. 43 13 331 for determining the crankshaft position of an engine in the event of failure of the crankshaft sensor has been found problematic. In particular in the lower speed range and during starting acceleration, the crankshaft rotation is subjected to relatively high dynamics which may not be detected by the camshaft sensor with sufficient precision. In the event of failure of the crankshaft sensor, very large angular errors may thus occur in determining the crankshaft position, which in turn may have adverse effects on the starting and driving performance of the vehicle. In an extreme case, the large firing angle errors may even cause critical operating conditions for individual engine components.